Mechanistic Study of Catalytic Hydride Reduction of −NO₂ to −NH₂ Using Isotopic Solvent and Reducer: The Real Hydrogen Source

The catalytic reduction of 4-nitrophenol to 4-aminophenol by using aqueous sodium borohydride (NaBH 4 ) solution is the most widely used model reaction to evaluate the catalytic activity of nanoparticles (NPs). It is generally believed that NaBH 4 is the hydrogen source for the −NO 2 to −NH 2 conversion. Water can also contribute hydrogen for the molecular conversion, but this potential hydrogen source has been overlooked since solvents are typically not involved in the chemical reactions. Surprisingly, through simple experiments using isotopic solvent (D 2 O) or reducer (NaBD 4 ), we find that the reduction reaction cannot proceed without the presence of water or other protic solvents. The vibrational spectroscopic analysis of the final reduction products also confirms that the hydrogen atoms in this conversion originate from the protic solvent. Therefore, it is water (or any protic solvents) rather than the hydride reducer that provides the hydrogen for the formation of the 4-aminophenol product. A reaction mechanism for the hydride reduction of 4-nitrophenol is proposed according to this finding. This work highlights the critical role of protic solvents in heterogeneous catalysis and provides a new perspective for understanding interfacial reactions.